Muscle Force Length Curve

Your muscle force length curve is a big answer to many questions of “why” during a bike fit.

“Why is this my saddle height?”

“Why does my back hurt?”

Your muscles have an ideal length to produce the most force. If you picture yourself in the bottom of a squat, that’s the hardest part in the lift. In the bottom of the squat movement, your gluteus maximus, the major force producer in the lift, is at its greatest length in the movement where it can’t produce as much force. When you’re at top of the squat, your glute is in the ideal force producing length. If you look back at the graph with that squat motion in mind and thinking about when your glute is most lengthened in the pedal stroke.

If we consider all the musculature in a pedal stroke and aim to have each muscle in the most ideal length for force production we can start to put together a good fit. If your saddle is too low, you’re leaving some glute force in the down stroke on the table. If you’re too high, your hamstrings are too lengthened to produce force to kick the pedal back around. Furthermore, if you’re too high that hamstring is being stretched every pedal stroke and your hamstring is connected to your hips and lower back and across the knee so everything is being pulled on every pedal stroke. We put together ideal knee angles and hip angles and know where the muscles can produce the most force to maximize efficiency in your pedal stroke.

Comfort vs Aerodynamics

There is a balance to be struck. You have to be able to make power on a bike whether it’s in a time trial or riding home from the grocery store. In all types of riding, wind resistance is the greatest force to overcome. The specific measurements to understand comfort and aerodynamics are pretty tricky to measure. To measure aerodynamics you really need a wind tunnel. For comfort, we rely on objective rider feedback after all other fit principles are utilized.

If we get the balance wrong, you can be very aerodynamic and even go pretty fast but you will feel more worn out on longer rides. If you go full comfort mode, you will have a very high CdA, or drag coefficient, and you can make a ton of power and get passed by a rider doing less watts who is more aero.

Don’t be fooled by a fitter without a wind tunnel offering an aerodynamic fit. Ask them how they are measuring drag and how they can offer aerodynamic measurements. While getting into the wind tunnel is beneficial, its very expensive and there are common practices and positions that we know are faster, and we can measure frontal area to give a best guess on aerodynamics. A good fitter that has a tri or time trial athlete will also look at cable housing exposure and make sure the little things are done first for aerodynamic gains.